Resuing Django example:
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
How to I obtain all Articles that do not have publications?
Article.objects.filter(...)
Specify publications
as None:
Article.objects.filter(publications=None)
Example:
>>> p1 = Publication.objects.create(title='1')
>>> p2 = Publication.objects.create(title='2')
>>> a1 = Article.objects.create(headline='a')
>>> a2 = Article.objects.create(headline='b')
>>> a3 = Article.objects.create(headline='c')
>>> a1.publications.add(p1)
>>> a1.publications.add(p2)
>>> a3.publications.add(p1)
>>> Article.objects.filter(publications=None)
[<Article: Article object>]
>>> _[0].headline
u'b'
Article.objects.filter(publications=None)
在“文章”模型中定义“ blank = True with Publications”字段,否则它将不允许您创建没有出版物的文章。
publications = models.ManyToManyField(Publication, blank=True)
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